Update pytest-cov requirement from <6.0,>=2.12 to >=2.12,<7.0

Updates the requirements on [pytest-cov](https://github.com/pytest-dev/pytest-cov) to permit the latest version.
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v2.12.0...v6.0.0)

---
updated-dependencies:
- dependency-name: pytest-cov
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

pyproject.toml

@@ -25,7 +25,7 @@ urlsketch = "sourmash_plugin_directsketch:Download_and_Sketch_Url"
 [project.optional-dependencies]
 test = [
   "pytest>=6.2.4,<8.4.0",
-  "pytest-cov>=2.12,<6.0",
+  "pytest-cov>=2.12,<7.0",
   "pytest-xdist",
 ]